The invention relates to measuring, in a non-invasive, non-surgical fashion, a vascular function that provides an indication of the expansibility of an artery.
Arteriosclerosis is the most common disease of the circulatory system, and is also the primary cause of a number of other serious illnesses. As arteriosclerosis progresses, qualitative changes and thickening of the material constituting the arterial walls lead to decreased expansibility (the rate of change of the volume, or cross-sectional area, of the arterial lumen with respect to an increase or decrease in internal pressure) of the arteries with respect to internal pressure (i.e., blood pressure).
When arteriosclerosis is advanced, various characteristic symptoms can be observed. Based on these symptoms, a number of methods to estimate the degree of the sclerosis have been devised. These methods include measuring the speed at which pulse waves propagate through the artery, measuring the acceleration of such pulse waves, and monitoring a spectrum of so-called Korotkov sounds. All of these methods involve the detection of changes in particular properties of a signal, where those changes correspond to changes in the dynamic characteristics, such as expansibility, of an artery.
Expansibility of arteries has been measured in animals through experiments in which the internal volume of an extracted artery is measured while the pressure within the artery is gradually increased or decreased, and has also been measured in humans through similar experiments performed subsequent to autopsies. For example, FIG. 1 shows the pressure-to-volume curve for the femoral artery of a dog over a range of negative and positive pressures. As shown in FIG. 1, sufficient negative pressure causes the artery to buckle, and thereby close completely, while increasing positive pressure eventually causes the artery to become fully expanded so that increasing pressure causes minimal increases in volume.
Because the shape of the pressure-to-volume curve changes as arteriosclerosis progresses, production of such a curve in a clinical setting would offer a significant diagnostic tool. Though direct measurement of the cross-sectional area (or volume) of an artery in vivo is difficult, indirect methods, such as ultrasound imaging or angiography, have been used. However, in a live subject, the blood pressure within an artery is readily variable over only a limited range (i.e., between the systolic and diastolic pressure). Thus, the full pressure-to-volume curve cannot be produced directly for a living subject.